Heretofore, significant advances in transparent armor have been attained by the geometrical arrangement of commercially available materials such as hard glasses, chemically-tempered glasses, thermoset biaxially stretched acrylics, polycarbonates, etc. Utilizing these material composites with areal densities of 10 to 11 pounds have attained merit ratings of about 1.40 against caliber .30 BALL M2 projectiles at 0.degree. obliquity, while, caliber 30 AP M2 projectiles have been defeated at 0.degree. obliquity utilizing these materials with areal densities of approximately 20 lbs to show merit ratings of 1.10 to 1.36. These composites have areal densities which are unacceptable to military requirements, particularly for aircraft applications, and for that reason have not been incorporated in armor systems. Further, transparent composites that defeat caliber .50 BALL M2 and AP M2 projectiles are nonexistent, yet non-combat vehicles are increasingly subjected to these threats.
Present day combat has necessitated a change in tactics to cope with guerilla warfare and has resulted in stringent requirements affecting the design of air and land vehicles. Non-combat vehicles are often subjected to spasmodic combat conditions and thus now receive armor considerations. The role of the helicopter as a combat vehicle has stimulated concern of its vulunerability to small arms fire, which includes engines, fuel tanks, gear drives and crew personnel. With mobility being a prime requisite for the present day army, the ability to navigate, which depends on visibility becomes increasingly important. Yet, the development of suitable and efficient transparent protective material lags greatly behind that of opaque armor.
However, nearly all transparent materials have been deficient in that they have only a limited capability for inducing fracture of small arms armorpiercing projectiles, which is the criterion in determining favorable ballistic performance. Attempts to obtain stress-wave reinforcement of stresses in the projectile by variation of composite glass frontal layer thickness have been singularly unsuccessful. It appears that available glasses do not have sufficient strength or modulus to induce high amplitude stress pulses in the projectiles.
It is therefore, the object of this invention to provide a transparent composite (ceramic-glass-plastic), capable of defeating caliber .30 AP M2 projectiles at 0.degree. obliquity, with 8 to 10 pounds areal density and caliber .50 AP M2 projectiles with an areal density of 18-20 pounds, which would not only decrease the vulnerability of vehicles of all types but would establish a technological milestone in the field of transparent armor.
The means, by which the selection of materials by properties and proper geometric arrangement for maximum ballistic protection, are the result of intensive investigation of the pressure wave transfer through ceramic composites impacted by steel cored projectiles. Theoretical and experimental conclusions indicate the fundamental material properties, modulus of elasticity and density, are favorable to ballistic protection when a high elastic modulus and low density facing material is utilized ia conjunction with a low elastic modulus and low density back-up material.
Expressions employed herein are defined as follows:
Areal density: armor weight expressed in pounds per square foot of surface area ##EQU1## where both the experimental and standard armors have the same areal density (lbs./sq. ft.)
AP: armor piercing
V.sub.50 penetration: is a term used to indicate that there is a 50:50 chance of penetration
Elastic Modulus: is the ratio of stress to strain within the elastic range of a particular material and the elastic range is the limit to which a particular material may be subjected and still return to its original form or shape.
This invention attains its stated object by the formation of a transparent composite consisting of a single ply transparent ceramic face plate,since the required property of high elastic modulus is more characteristic of ceramics than glass, and also of low density with the general overall characteristic of being relatively hard and brittle, while thee transparent back-up layer or layers is of low elastic moduli, of low density and also of relatively tough resilient material. The face plate and the back-up plate of one or more layers are bonded together by any suitable transparent adhesive to form the desired composite.